check valve orientation?

[westtexasbaja]

I'm installing an inline check valve on a fuel tank vent. Whats the best orientation? Horizontal? Or vertical with the tank sucking down on the valve (vent line to bottom of valve)? This seems flow the best. It does not seem to flow well vertically, rotated 180, with the tank trying to suck UP (vent line to top of valve). Does that make sense......?

Thanks,
G.